There is a small-scale power system, known as a microgrid or the like, which supplies power to customers in a specified region using multiple types of distributed power source installed in the specified region, including engine generators, turbine generators, power storage systems, fuel cells, and photovoltaic generation, wind-power generation, or other natural energy generation apparatus.
When a small-scale power system is connected to a commercial power system of an electric power company, there includes either one of two methods, i.e., one: forward power flow which involves receiving power from the commercial power system, and the other: reverse power flow which involves supplying power to the commercial power system. In each method, it is required to perform power supply and demand control of distributed power sources in the small-scale power system in order to keep the electric power flow (electric energy) at any given location in a power system constant or keep deviations between load power and power output in the small-scale power system constant.
To realize power supply and demand control of multiple distributed power sources, it is necessary to estimate load power, calculate such total power output of the distributed power sources that will keep deviations between the estimate load power and power output constant and to assign the total power output as power output targets among the distributed power sources.
For example, Patent Document 1 (Japanese Patent Laid-Open No. 2004-48852) discloses a power generation planning method for planning an amount of power to be generated by each generator set for electricity supply from a generator-set group made up of a plurality of generator sets to a consumer group of one or more consumers in such a way as to optimize predetermined evaluation values while satisfying limiting conditions that estimated total received power of the entire consumer group will be accommodated and that total reserve capacity for a total amount of power planned to be generated by the entire generator set group will be equal to or higher than predetermined total reserve capacity.
Further, if a small-scale system includes a natural energy generation apparatus whose power output is affected by insolation, wind conditions or other natural conditions, it is necessary to calculate the total power output of distributed power sources other than the natural energy generation apparatus by taking into consideration an estimated value of the power output and estimated value of load power of the natural energy generation apparatus.
For example, Patent Document 2 (Japanese Patent Laid-Open No. 2004-289918) discloses a method for realizing power generation estimation based on weather information. The method estimates amounts of power to be generated by photovoltaic generation and wind-power generation in each area, a day ahead or in real time, by gathering weather information concerning sunshine and wind force.
Power output command values are issued to distributed power sources to perform power supply and demand control in order to keep the electric power flow (electric energy) at any given location in a power system constant or keep deviations between load power and power output in the small-scale power system constant. Specifically, the total power output of the distributed power sources in the target system is calculated based on the estimations of loads and the power output of the natural energy generation apparatus, and a power output command value is determined for each distributed power source using economical load dispatching (hereinafter abbreviated to ELD) so as to achieve greater economy and realize the total power output.
Rules used to keep the electric power flow at any given location in a power system constant include a rule known as supply-demand balancing which involves evaluating how actual electric energy deviates from target electric energy in a given amount of time, and it is necessary to control output of the distributed power sources in the small-scale power system so as to achieve required supply-demand balancing.
In small-scale power systems, as in the case of medium- or large-scale power systems, if there is any difference between the estimated values and the actual values of the load power (actual values) or between the estimated values and the actual values of the power output of the natural energy generation apparatus, it is necessary to achieve the supply-demand balancing at any given location by modifying power output commands of the generation apparatus which lend themselves to power control, so as to eliminate the difference.
Performance in the supply-demand balancing is evaluated in terms of how quickly the supply-demand balancing can be achieved when any difference arises between the estimated values and the actual values of load power (actual values) or between the estimated values and the actual values of the power output of a natural energy generation apparatus, and it is known that large electric power companies generally supply electric power by achieving 30-minute supply-demand balancing.
However, small-scale power systems such as microgrids are subject to large fluctuations in the power generation due to reliance on the natural energy generation apparatus and because of limited demand. Therefore, it is more difficult for the small-scale power systems to achieve the supply-demand balancing in short time periods than for medium- or large-scale power systems. Thus, the small-scale power systems such as microgrids require a power supply and demand (supply-demand) control apparatus and power supply and demand (supply-demand) control method with the supply-demand balancing performance higher than those for the medium- or large-scale power systems.